This invention pertains generally to metering active electrical energy by solid state electronic techniques and, in particular, in one aspect relates to an improved multiplier and method for multiplying analog signals representative of current and voltage in the electrical system whose energy is being metered.
Electrical energy (kWh) has been and continues to be metered with the familiar rotating disc-type of meter. In addition, the instrumentation and metering arts include proposed systems having apparatus employing electronic and solid state devices for measuring power and energy. In such apparatus, the electronic and solid state devices replace the conventional rotating disc.
Further, there is disclosed and claimed in applicant's copending applications referred to above, apparatus and methods for metering electrical energy in an electrical system which involves producing analog signals from line currents and voltages. Pairs of analog signals, representing current and voltage variables, are processed in time-division multiplier networks which multiply the analog signals to produce a series of width and amplitude-modulated pulse signals, each representing instantaneous partial power. Pulse signals from different multiplier networks are summed to provide another series of pulse signals, each representing instantaneous total power. The series of pulse signals representing instantaneous total power are processed through a low-pass filter to produce another signal representative of average total power in the system. Subsequently, the signal representing average total power is processed in an analog-to-pulse rate converter which produces a series of output pulse signals, each representing a quantized amount of electrical energy. A stepping switch and register perform conventional accumulation, storage and display functions in response to the series of output pulse signals delivered thereto.
The multipliers used in the above-referenced copending patent applications are described therein as feedback-type up-down integration multipliers, which combine the analog input signals to produce a series of output pulses in accordance with a sampling frequency with the output pulses being amplitude and width-modulated in accordance with the input analog signals. As known to those in the art, such feedback type of multipliers inherently produce phase shift errors in the pulse with and heighth-modulated output signals, with the amount of phase shift errors being related to the rate of the sampling frequency compared to the frequency of the input analog signals.
At an ordinary sampling frequency of 10 kHz a pulse-width-amplitude multiplier phase shift is about 1.5.degree. at a 60 Hz signal frequency. This phase shift of 1.5.degree. can result in a multiplier error of about 5 percent if the phase shift between two input analog signals being multiplied amounts to 60.degree. (power factor equal to 0.5).
Accordingly, it is one object of the present invention to provide a method and apparatus for use in a solid state electrical energy metering system for multiplying electrical analog signals representing voltage and current in the system to produce a series of amplitude and width-modulated pulses without the introduction of phase shift errors.
It is another object of this invention to provide such a multiplier apparatus and method utilizing solid state circuitry which may be fabricated in the form of monolithic integrated structures.
It is another object of this invention to provide an improved triangle wave generator and method for use with the multiplier apparatus and method of this invention.
Briefly, in accordance with one embodiment of the invention, there is provided an electronic kWh meter which includes means for developing pairs of analog signals representative of currents and voltages in the electrical system being monitored. The current and voltage analog signals in a pair are multiplied in a direct-type pulse modulator, which does not generate any internal phase shift, and which functions to multiply the two analog signals and produce a series of width and amplitude-modulated pulse signals, each representing instantaneous power. If the electrical system being monitored is a polyphase system, so that a plurality of multipliers are provided, then the pulse signals from the different multiplier networks are summed to provide another series of pulse signals, each representing instantaneous total power. The series of pulse signals representing instantaneous total power are processed through a low-pass summing filter which produces another signal representative of average total power in the system. The averge total power signal is processed in a quantizing analog-to-pulse rate converter which produces a series of output pulse signals each representing a quantized amount of electrical energy. A stepping switch and register perform conventional accumulation, storage and display functions in response to the series of output pulse signals delivered thereto.
Other objects, as well as various features of the invention, appear hereinafter from the description of the preferred embodiments taken in due consideration in connection with the drawings.